The present invention relates to the field of circuits and, more particularly, to a method and apparatus for current recycling for circuits.
Current steering is gaining high visibility as a vehicle for high speed data transfer. The constant need to transfer more information faster, accompanied by increases in data processing capability has necessitated an expansion to data transfer rates considerably higher than what was previously possible. One data transmission scheme used to accomplish these higher rates is differential data transmission in which the difference in voltage levels between two signal lines form the transmitted signal. Differential data transmission is commonly used for data transmission rates greater than 100 Mbps and over long distances. Noise signals in the differential transmission shift the ground level voltage and appear as common mode voltages. Thus, the deleterious effects of noise are substantially reduced.
An example of a conventional low voltage differential swing (LVDS) driver circuit 800 used for differential data transmission is shown in FIG. 10. The difference in voltage between the output signals OUT+, OUTxe2x88x92 on the output terminals 803, 805 form a pair of differential signals. A pair of differential signals, means two signals whose current waveforms are out of phase with one another. The individual signals of a pair of differential signals are indicated by reference symbols respectively ending with xe2x80x9c+xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d notation (e.g., S+and Sxe2x88x92). The composite notation xe2x80x9c+/xe2x88x92xe2x80x9d is employed to indicate both differential signals using a single reference symbol, e.g., S+/xe2x88x92.
The LVDS driver circuit 800 includes a direct current (DC) constant current source I1 coupled to voltage supply VDD, four n-channel metal oxide semiconductor (NMOS) switches M11-M14, and a resistor R1 coupled between the common node COM and voltage supply VSS. The four transistor switches M11-M14 are controlled by input voltage signals VIN1, VIN2 and direct current through load resistor Rt as indicated by arrows A and B. The input voltage signals VIN1, VIN2 are typically rail-to-rail voltage swings. The gates of NMOS switches M11 and M14 couple together to receive input voltage signal VIN1. Similarly, the gates of NMOS switches M12 and M13 couple together to receive input voltage signal VIN2.
Operation of the LVDS driver circuit 800 is explained as follows. Two of the four NMOS switches M1-M14 turn on at a time to steer current from current source I1 to generate a voltage across resistive load Rt. To steer current through resistive load Rt in the direction indicated by arrow B, input signal VIN2 goes high turning ON NMOS switches M12 and M13. When input signal VIN2 goes high, input signal VIN1 goes low to keep NMOS switches M11 and M14 OFF during the time NMOS switches M12 and M13 are ON. Conversely, to steer current through resistive load Rt in the direction indicated by arrow A, input signal VIN1 goes high and is applied to transistor switches M11 and M14 to make them conduct. Input signal VIN2 goes low to keep NMOS switches M12 and M13 OFF during this time. As a result, a full differential output voltage swing can be achieved.
In other words, the prior differential LVDS driver circuit 800 sources current from VDD and sinks this current to VSS. In between, a switching array controlled by a Boolean signal routes the source current through the interconnect to an external resistive load. The return current is routed back to the array and sunk to VSS. The polarity of the voltage drop across the resistor carries the digital value. Changing the polarity of the array causes the current to route through the interconnect and resistor in the opposite direction and transfer the other digital value.
In addition to the requirement for transferring data at greater speeds, there is continuous pressure to reduce the amount of power consumed by integrated circuits. The differential LVDS offers reduced power consumption as compared with other drivers. The disadvantage is the need to have a set of interconnects for each signal because a differential LVDS driver is provided for each of the interconnects. It is this need to have a set of interconnects for each signal that drives the power of an integrated circuit using differential LVDS driver circuits. Thus, it would be desirable to provide a means to implement the differential LVDS driver circuits at a lower power.
The present invention is directed to a process and circuitry for drawing current from a power source to produce a first operation, such as a Boolean operation, and reusing the current to generate a further operation. In other words, the current from the first operation may be used to perform a second or subsequent operation instead of sinking the current to ground or another power source. This process is referred generally herein as current recycling.